1. Field
The disclosure relates to a sniffing leak detector according to the reference measurement principle, comprising a basic unit including a vacuum pump device, further comprising a handpiece provided with a sniffing tip and being connected to the basic unit via a hose, a measurement-gas suction opening for sucking in measurement gas, a reference-gas suction opening for sucking in reference gas, and a gas analyzer for determining the concentrations of the measurement gas and the reference gas.
2. Discussion of the Background Art
WO 00/55603 describes a sniffing leak detector according to the reference measurement principle comprising, apart from the measurement-gas suction opening, also a reference-gas suction opening. Sniffing leak detection is frequently performed on test objects which contain cooling agents or hydrocarbons. In such a process, the media present in the test object will serve as a test gas. In case that a leak exists, small quantities of the respective test gas will reach the sniffing leak detector. The sniffing leak detector includes a gas analyzer adapted to detect the test gas. A problem in sniffing leak detection resides in that there will be suctioned not only the test gas issuing from the leak but also gas from the ambience of the sniffing tip. If the latter gases from the ambience should happen to include minor concentrations of the test gas, which may originate from leaks detected at an earlier time or from the filling station of a production line, also these test gas concentrations will be registered by the gas detector. In order to differentiate between measurement gas and ambient gas, it has been proposed to provide a measuring cuvette and a reference cuvette in the gas analyzer, thus obtaining a measurement gas signal and a reference signal. Both signals are processed in the lock-in amplifier to the effect that the respective useful signal will first be subjected to a modulation and then to a phase-sensitive rectification. Obtained in this manner is a useful signal which represents the difference of the measurement signal over the reference signal.
WO 02/48686 A2 describes a sniffing leak detector which again comprises a measurement-gas suction opening and a reference-gas suction opening. With the aid of a switching valve, both suction openings will be alternately connected to the inlet of a cuvette arranged in connection to a vacuum pump device.
In refrigerators and air-conditioning systems, increased use is made of CO2 as a cooling agent. In tightness testing performed on such devices, the ambient air has to be checked for CO2 as a test gas. CO2 is also contained in a high concentration in the air exhaled by the operating person. In the usual sniffing leak detectors operating according to the reference measurement principle, the measurement-gas suction opening is arranged at the end of the sniffing tip whereas the reference-gas suction opening is arranged laterally and at a recessed position. Therefore, it may occur that a stream of the operating person's exhaled air will reach the reference-gas suction opening. In such a case, the device will detect a “negative leak” because the concentration of CO2 in the ambient air around the measurement site is higher than at the measurement site itself.